This invention relates generally to cementitious compositions and, more particularly, to an improved composition particularly useful as a pavement patching compound.
It is known that a major factor in the long term deterioration of concrete is the formation of ettringite. This occurs as a result of the reaction of tricalcium aluminate (3CaO.Al.sub.2 O.sub.3) (abbreviated C.sub.3 A) present in Portland cement with sulfate. The ettringite causes an increase in the volume or length of the concrete which, of course, results in splitting, cracking, and even crumbling. While mixtures of Portland cement and calcium sulfate hemihydrate (commonly called alpha gypsum) have previously been used and are in some cases advantageous, the interaction between the C.sub.3 A and the sulfate from alpha gypsum has greatly limited the use of such mixtures. Their application has been primarily as "quick patches" the long term durability of which has been notoriously poor. To reduce ettringite formation in such mixtures and thus improve concrete life and durability others have proposed utilizing Portland cement with low tricalcium aluminate content. Such an approach is the subject of U.S. Pat. No. 3,852,081, the disclosure of which is expressly incorporated herein by reference.
It is also known that some types of cements containing pozzolan offer superior sulfate resistance. It has not heretofore been known, however, that a combination of moderate to high C.sub.3 A containing cement together with alpha gypsum and pozzolan would offer a product having properties equal to or exceeding those cements having low tricalcium aluminate and alpha gypsum. The result is particularly unexpected because the sulfate present with high C.sub.3 A content cement would be expected to show poor durability.